How do you create a slice of ocean with an animated surface in blender 2.8?
Add a Plane and subdivide it couple of times
Select its border and add it to a new vertex group


Add all modifiers to the stack, pay attention to green highligts




Ocean should be Displace instead of Generate in order to utilize vertex group. 

Vertex group later is displaced strictly on Z axis down below and cut by Cube via boolean Intersect.

Adjust Cube's scale as you need and enable Autosmooth (80 deg) in Plane settings to have a sharp corner at the cut.